1. Field of the Invention
The invention relates to a method for processing ceramic materials. In particular, this invention relates to a method for heating ceramic materials involving the use of electromagnetic energy and optionally firing ceramics involving the use of electromagnetic energy in combination with conventional radiative/convective heating, and more particularly to a method for controlling the power dispersion of the electromagnetic energy by density selection of the pieces to uniformly distribute the heating energy throughout the ceramic material.
2. Technical Background
Conventional heating used in the manufacturing of ceramic materials typically utilizes radiative gas or electric resistance heating. Utilization of conventional radiative/convective heating typically results in a thermal differential within the ceramic material. This differential is due, in part, to the fact that radiant heating is applied only to the surface of the material and it relies on the thermal conductivity of the material, typically poor, to transmit the thermal energy beneath the surface and into the interior or core of the piece. In other words, conventional heating involves heat transfer that is predominantly achieved by radiation or convection to the surface followed by conduction from the surface into the interior of the ceramic body. If a core-surface thermal differential develops that is too great, internal cracking, external cracking, and distortion of the ceramic material can occur. Fast drying or firing further exacerbates this problem of poor heat transfer, and ultimately cracking.
Additionally, the presence of a core-surface thermal gradient can also result in uneven sintering, specifically surface sintering prior to, and at a faster rate than, interior sintering. As a result, the ceramic material may exhibit non-uniform properties.
Solutions to these problems which have been proposed involve reducing the rate of heating or allowing lengthy holds at certain temperatures. Each of these solutions allows heat energy to be conducted into the core of the ceramic material, which in turn, allows the temperature of the core of the ceramic material to xe2x80x9ccatch upxe2x80x9d with that of the surface, thereby minimizing the surface/core temperature differential. Unfortunately however, the theoretical limits of conventional radiative or convective heating typically result in slow heating rates for all ceramic materials, the exception being ceramic pieces exhibiting small dimensions.
Microwave heating of ceramics has alternatively been successfully used to both dry and assist in firing ceramic materials. In comparison with conventional heating, microwave heating involves depositing energy directly within the ceramic material in accordance with a volumetric heating mechanism. More specifically, the utilization of microwave energy involves delivering a uniform application of the energy to the entire cross section of the ceramic article, rather than to the article surface. Although microwave heating of ceramic materials is much faster than conventional radiant heating because of this volumetric heating, it, like radiative heating, results in the ceramic article exhibiting a thermal differential; albeit an opposite thermal differential with the core of the ceramic article exhibiting a higher temperature than that of the surface. Specifically, as the ceramic materials, typically poor absorbers of microwave energy at low to intermediate temperatures, are heated with microwaves at high temperatures, the interior of the ceramic article very rapidly begins to absorb substantial amounts of microwave energy; this effect is known as thermal runaway. Although the surface is heated along with the core of the ceramic material, the surface rapidly loses much of its heat energy to the surroundings, which is typically cooler than the average ceramic material temperature. As the core starts to preferentially absorb the microwave energy this thermal runaway phenomenon becomes self-propagating. Simply stated, as the temperature of the ceramic material increases, the heat losses become greater, and the magnitude of the core-surface thermal differential increases, again leading to thermal stress within, and ultimately cracking of, the ceramic article.
In addition to heat losses from the surface of the ceramic article, non-uniformity of the microwave distribution within the dryer, kiln, furnace, or oven, and non-uniform material properties of the ceramic article lead to differential absorption of the microwave energy by the ceramic article, and contribute to the microwave heating thermal differential.
In the processing of cellular ceramic products, the as-extruded piece is subjected to several steps in which the piece is dried and fired, separately. All steps have specific time-temperature cycles in which the heating rates, hold temperatures, and hold times are all important to the formation of the required physical properties of the body. Using conventional hot air techniques, it can take longer to produce relatively larger parts. Therefore, depending upon the size of the part substantial lead time may be required for delivery of a product in the best of circumstances.
In an effort to alleviate this concern prior methods include the use of a combination of microwave energy and conventional heating techniques (resistive, gas firing, etc.) to process cellular ceramics from extrusion through the firing using one thermal process. This includes drying and firing, and eliminates the handling step (or steps, where the parts are dried twice) between dry and fire. The process can be applied to other cellular ceramic products as well.
Hybrid microwave/conventional heating or microwave assisted heating has been utilized as an alternative to overcome the problems of conventional radiative and microwave-only heating. In microwave assisted heating involving both microwave and radiative/convective heating, the volumetric heating provided by the microwaves heats the components, while the conventional radiative/convective heating provided by gas flame or electric resistance heating elements minimizes heat loss from the surface of the components by providing heat to the surface and its surroundings. This combination or hybrid heating can result in heating that avoids thermal profiles associated with conventional and microwave-only heating. As a result, thermal stresses can be reduced and or minimized and thus the ceramic articles can be heated more rapidly.
Conventional dielectric drying processes and gas firing can be combined in one thermal process by using microwave energy to assist in drying and firing parts faster and with less handling. Microwave drying works on the same principle as do the dielectric dryers, but is of a higher frequency and can be run more efficiently. Microwave assisted firing can reduce thermal gradients through a part during firing, allowing faster heating ramps, usually cutting ramp times by 50% or more of conventional gas firing.
In drying a wet piece, volumetric heating specifically aimed at polar molecules (i.e., water) is a great advantage over conventional methods of drying. This is how current dryers work. The advantages of using microwave drying are two fold. The high frequency of microwave energy allows the use of lower wattage and more efficient drying, while the actual apparatus has a smaller footprint. Also, unlike dielectric dryers, a microwave energy source can be used to assist in firing ceramics. A thermal process set to dry and fire parts would require no handling from the dryer to the kiln, and no cooling and re-heating steps either.
While microwave energy alone can be used to heat cellular ceramics, a much more efficient and reliable method is to meld the current technology in gas fired kilns with microwave assisted heating, creating a hybrid kiln capable of fast firing. Green ware is made up of organic and inorganic materials, and they react in different ways as they are subjected to the time-temperature cycle of firing. The organic materials burn in the presence of oxygen at certain temperatures, while the inorganic materials contain chemically bound water that is driven off.
The two chemical processes are often at odds with each other. The release of heat in the exothermic reaction of the organic binders, and the heat requirement of the endothermic chemically bound water removal cause thermal gradients resulting in thermal/mechanical stress in the parts. The burning of the organic material requires the kiln to be able to extract the heat fast enough so that the core of the piece does not over heat. The removal of chemically bound water requires the kiln to supply the parts with enough heat to prevent a cool core compared to the skin.
During processing, an even power distribution of the microwave energy is important to obtaining uniform properties. For example, during the endothermic removal of chemically (firing) or physically (drying) bound water high amounts of microwave power are used. When the power is not evenly distributed then some wares, or sections of a ware, will receive too much energy, while others may not receive enough. Either case can result in cracking or non-uniform characteristics.
Most cellular ceramic substrates are fired in a fossil fuel tunnel or periodic kiln. Fossil fuel combustion has been the heating method of choice because it offers not only radiative heating, but also convective heating due to the velocity of the flame and products of combustion. Even the utilization of these two heating modes is most often not effective enough to overcome the thermal differences within the piece. Because they are applied only to the surface they must rely on thermal conductivity of the body material to effect the temperature from the skin to the center of the piece. This is exacerbated by the fact that cellular ceramics are by nature of their material and geometry, thermally insulating.
For the reasons mentioned above, as much as 50% of a firing cycle can be simply the time required for heating a piece to the holding temperatures. Microwave radiation will heat an object volumetrically (i.e. the whole part receives the radiation at the same time), and can drastically reduce the amount of time consumed in heating a piece to the hold temperatures by reducing or substantially eliminating the thermal gradient across a piece. The thermal gradients induced by heating a piece too quickly from the outside to the inside can cause cracking, and undesirable or non-uniform properties.
The benefits of volumetric heating extend beyond simply shortening the time requirements for heating ramps. It can also be employed during reactions to control the rate of the reaction and ensure uniformity during phase changes, chemically bound water removal, debind, and sintering. For example, if the piece is entering an endothermic regime where chemically bound water is being removed, the skin of the part will consume most of the energy available, leaving the core cold, and the skin shrinking. This causes not only thermal gradient stresses, but also mechanical stress related to differential shrinkage. Using a combination of conventional and microwave energy, heat is distributed to the core and skin and the stress mentioned above during these critical regions of the firing cycle are negligible. It is not difficult to imagine the benefits during other parts of the cycle, such as sintering.
Similar phenomenon are known to occur during drying. For example, as the ceramic ware dries it loses its elasticity and/or shrinks. If there are any differential stresses caused by shrinkage the body is susceptible to cracking. It is therefore important to balance the evaporation of water from the outside surface with that of the removal of water (evaporative or osmotic) from the center of the ceramic ware.
Microwave energy of frequencies supplied by inexpensive and reliable magnetrons are commercially available, affordable, and suitable for processing cellular ceramics. The technology in which hybrid gas firing and microwave heating are combined is known.
Although, various methods of implementation have been proposed, it can be difficult to coordinate the respective microwave and conventional energy inputs to achieve optimal uniform heating of the ceramic article. Variations on microwave-assisted ceramic firing standard control methods are disclosed in PCT Applications WO 95/05058 and WO 93/12629 and U.S. Pat. No. 5,191,183. These documents generally disclose methods of independently controlling the quantities of heat generated in the ceramic article by the microwave energy and radiant heat by measuring the ambient temperature within an enclosure containing the ceramic article. Based on, and in response to, this ambient temperature measurement, the heat generated in the ceramic article is controlled by one or both of the microwave energy or radiant heat. Although this type of control method is an improvement over prior conventional control methods, the non-uniform mixing of kiln gases and the effects of chemical reactions that occur within the ceramic material make it difficult to accurately predict the ceramic article surface and internal temperatures.
Heating uniformity is of paramount importance in most industrial heat treating applications. Typically, multiple ceramic articles are placed within the kiln to increase productivity. This increases the importance of uniform distribution of suitable amounts of thermal energy within the kiln to assure that each ceramic piece is fired properly, thus avoiding burning, cracking or other undesirable results. One problem encountered in treating multiple articles with microwave energy is known as the boundary effect. This effect tends to cause an uneven power distribution of microwave energy directed to the boundary of the article, the interface of the ware with its surroundings (generally the atmosphere in the kiln or dryer).
The art lacks a solution capable of providing to multiple pieces uniform power dispersion within each piece, general applicability to a wider variety of sample compositions (although during any one firing the composition is substantially the same), a variety of ware sizes and geometry, a better pore size distribution, increased strength and thermal shock resistance, decreased coefficient of thermal expansion and eliminating internal and external cracks.
Also lacking is a process that provides increased throughput (shorter time-temperature cycle) during critical regions which were previously slowed down significantly because of the inefficiencies associated with surface heating from the combustion heating process.
Accordingly, it is an aspect of this invention to provide a method for heating a plurality of ceramic bodies, including:
a) providing ceramic-forming raw materials and blending the raw materials with an effective amount of vehicle and forming aids to form a plastic mixture therefrom and thereafter forming the plastic raw material mixture into a plurality of green bodies;
b) placing each one of the plurality of green bodies in proximity to an adjacent one of the plurality of green bodies such that upon heating with electromagnetic waves each green body is subject to no more than about 1.5 times the power density at the boundary than in the bulk thereof; and
c) drying the green bodies utilizing energy in the form of electromagnetic waves.
According to another aspect of the present invention, when the ceramic is a honeycomb cellular cordierite body the method further includes heating the green body up to a maximum temperature of between about 1360xc2x0 C. and about 1435xc2x0 C. to produce a fired body that is predominantly cordierite, wherein the firing includes utilizing a combination of microwave and convective or radiative heating during periods where the green body is subject to an endothermic reaction or phase transition.
According to another aspect of the present invention, the firing of a honeycomb cellular cordierite body further includes placing each one of the plurality of green bodies in proximity to an adjacent one of the plurality of green bodies within a firing chamber such that upon heating with electromagnetic waves each green body is subject to no more than about 5 times the power density at the boundary than in the bulk thereof.